Thermal batteries are designed for immediate and short-duration activation under extreme operating conditions. In an inert state suitable for storage, a thermal battery is dormant, and can remain inactive for long periods of time. Upon initiation, a thermal battery instantly activates to serve as an accurate voltage source that is stable for a predetermined time duration.
Contemporary thermal batteries include an anode and cathode separated by a solid electrolyte. In a solid state, the electrolyte is dormant, and serves as an electrical buffer between the anode and cathode. When converted to a molten state, for example by means of heat produced by an activated pyrotechnic charge, the electrolyte becomes a conductor, serving as a conduit between the anode and cathode. The thermal battery remains active for a predetermined period of time until the charge is exhausted.
Examples of thermal batteries are disclosed in U.S. Pat. Nos. 5,895,730 and 6,198,249, the contents of which are incorporated herein by reference. Such thermal batteries are limited in their operation in that they suffer from relatively low energy density, short-duration activation period, limited shelf life in storage, poor reliability under exposure to extreme acceleration, large size and weight, limited altitude operation range, and narrow temperature operation range.
The present invention is directed to a thermal battery and process for forming a thermal battery that overcome the limitations of conventional embodiments. In particular, the battery and method of the present invention are well suited for battery applications that require highly integrated thermal batteries that are relatively small in physical size, yet are capable of reliable performance over a wide range of operating conditions.
In one aspect, the thermal battery of the present invention is housed in a chamber that utilizes micro-electromechanical systems (MEMS)-based technology to offer superior chemical stability and advantageous mechanical and thermal properties. In another aspect, the thermal battery of the present invention is activated by heat, for example heat generated by a pyrotechnic charge, for immediate and thorough activation of the electrolyte. In another aspect, the anode, cathode and electrolyte may be formed of pellets having a curved interface for increased current density.
In another aspect of this invention, the pyrotechnic charge consists of a heating pellet including suitable chemical ingredients, which is utilized to provide rapid, controlled, high-temperature heating of the electrolyte to achieve rapid melting. In a preferred embodiment, the heating pellet consists essentially of thermite, a blended mixture of two solid components, iron (III) oxide and aluminum powder, that may be pressed and shaped as described hereinafter. Upon ignition, thermite produces a large quantity of heat (relative to the mass of the components) and two distinct solid-based byproducts (iron and aluminum oxide) with zero moles of gas. By xe2x80x9csolid-basedxe2x80x9d it is meant that the byproduct is a solid at ambient conditions. (The thermite reaction may initially produce molten iron.)
Because all of the thermite reaction byproducts are solid-based, i.e., no gases are evolved, all of the evolved energy (847.6 kJ/mole of energy) is available for heating the solid eutectic carbonate electrolyte of this invention. When gases are evolved as byproducts of a chemical reaction, there is a variability in the reaction kinetics, i.e. turbulence, which creates oscillations in pressure and heat output. This variability, in the case of a thermal battery, leads to uncontrolled or erratic melting of the carbonate electrolyte and to possible inefficiencies and interruptions in the generation of electrical power. Such limitations are avoided in this aspect of the present invention by using a suitable material, e.g. thermite, as the primary heat source.
In another aspect of this invention, the thermite pyrotechnic charge is activated by means of an ignition strip that burns at a high enough temperature to ignite the thermite. In a preferred embodiment, the ignition strip includes a fuse roll or foil strip consisting essentially of about 54 wt. % magnesium powder, about 30 wt. % Teflon(trademark), and about 16 wt. % Viton(trademark) (hereinafter xe2x80x9cMTVxe2x80x9d). Teflon(trademark) and Viton(trademark) are materials available from E. I. DuPont de Nemours and Company, Wilmington, Del. An MTV ignition strip is preferred to a simple magnesium strip for purposes of this invention because it has been found that the heat output from combustion of the MTV strip is much higher and more controlled. Also, an MTV ignition strip can be easily processed into the sizes and shapes required for use with the thermal batteries of the present invention.
Alternatively, an ignition strip in accordance with the present invention may consist essentially of bisnitro cobalt-3-perchlorate (BNCP), which is synthesized according to known techniques.
In another aspect of this invention, the electrolyte is in the form of a thin, solid tablet or pellet at ambient conditions, and is positioned between the anode and cathode elements of a cell unit. A preferred electrolyte in accordance with the present invention includes a three-component blended eutectic salt mixture selected to have a melting temperature in the range of about 490xc2x0 C.-520xc2x0 C. In a particularly preferred embodiment, the electrolyte consists essentially of one of the following two ternary eutectic mixtures of alkali carbonate salts.
A first preferred eutectic carbonate salt mixture consists essentially of lithium carbonate (Li2CO3), sodium carbonate (Na2CO3), and potassium carbonate (K2CO3), hereinafter abbreviated as xe2x80x9c(LNk)2CO3xe2x80x9d. In general, this mixture may include about 38-49 wt. % lithium carbonate, 26-37 wt. % sodium carbonate, and 20-30 wt. % potassium carbonate. For example, a preferred mixture of about 43.5 wt. % lithium carbonate, 31.5 wt. % sodium carbonate, and 25 wt. % potassium carbonate has been determined to have a eutectic melting point of 518xc2x0 C., within the preferred electrolyte melting temperature range.
A second preferred eutectic carbonate salt mixture consists essentially of lithium carbonate (Li2CO3), sodium carbonate (Na2CO3), and rubidium carbonate (Rb2CO3), hereinafter abbreviated as xe2x80x9c(LNR)2CO3xe2x80x9d. In general, this mixture may include about 34-44 wt. % lithium carbonate, 33-44 wt. % sodium carbonate, and 17-28 wt. % rubidium carbonate. For example, a preferred mixture of about 39 wt. % lithium carbonate, 38.5 wt. % sodium carbonate, and 22.5 wt. % rubidium carbonate has been determined to have a eutectic melting point of 499xc2x0 C., also within the preferred electrolyte melting temperature range.
In accordance with the present invention, it has been found that a ternary eutectic salt mix achieves superior performance in thermal battery applications as compared with single or two-component mixtures. In particular, it has been found that the heat capacity of the ternary eutectic mix is much higher than that for a two component (carbonate or non-carbonate based) eutectic molten salt. This ensures that the molten electrolyte salt in the ternary composition remains as a liquid melt for a much longer time, thus leading to the longer operation life and markedly improved electronic transfer.
In another preferred embodiment of this invention, the electrolyte consists essentially of a ternary inorganic alkali carbonate eutectic salt composition blended with a minor proportion, e.g., about 0.005%-10% by weight, most preferably about 1% by weight, of a surfactant to enhance electron mobility during electrolyte activation and to improve wetting of the molten electrolyte to the internal walls of a zeolite molecular sieve as hereinafter described. A particularly preferred surfactant for such purposes is sodium lauryl sulfate.
In still another aspect of this invention, an anode element of a thermal battery according to the present invention includes an alkali/alkaline earth metal alloy shaped as a lozenge or pellet. In a preferred embodiment, the anode element consists essentially of 15-25 wt. % of lithium and 75-85 wt. % of germanium, preferably about 20 wt. % of lithium and about 80 wt. % of germanium, pressed into thin foil of about 0.01-1.00 mm in thickness. In a further preferred embodiment, the anode foil is partially enclosed in a composite mixture consisting essentially of vanadium, metal carbonate salt electrolyte, and a zeolite composition bent or fabricated into the shape of a foil cup to act as a separator.
In still another aspect of this invention, a cathode element of a thermal battery according to the present invention includes a material having adequate electrical conductivity, structural integrity at the normal operating temperatures of the battery, and a low dissolution rate in molten carbonate, shaped as a lozenge or pellet. In a preferred embodiment, the cathode element(s) of the present invention consist essentially of vanadium pentoxide (V2O5) having at least some degree of porosity. Alternatively, vanadium trioxide (V2O3) or vanadium dioxide (VO2) can be substituted for V2O5 as the cathode for some embodiments of the present invention. Upon activation of a thermal battery according to this invention, heat is evolved and expansion of the cathode occurs. The pores/voids within the cathode become larger. Furthermore, the interface between the anode and the cathode at the higher temperature may undergo some separation. To avoid possible resultant leakage of the molten electrolyte, another embodiment of the present invention provides for at least partially enclosing the cathode in a separator as described above for the anode.
In a preferred embodiment of this aspect of the present invention, the separator element contains sodium aluminosilicates materials generally known as zeolites. Zeolites act (due to their porous structure) as molecular traps for the molten electrolyte. Historically, zeolites were limited to crystalline, porous aluminosilicate compounds. Other porous materials have been found to perform as well as the classical zeolite compounds, such that the current definition of zeolite encompasses materials beyond aluminum and silicate, and includes other materials that have well-defined porous crystalline structures.
In another aspect of this invention, a separator element is positioned between the heat pellet and/or the electrolyte pellet and the anode and/or the cathode, or between adjacent cells in a cell stack, to prevent leakage of molten electrolyte during activation. In a preferred embodiment, the separator consists essentially of a composite of vanadium and a zeolite-type molecular sieve. The structures and uses of zeolites are generally described in xe2x80x9cZeolite Molecular Sieves,xe2x80x9d by Donald W. Breck (John Wiley and Sons 1974), which text is incorporated herein by reference. A preferred zeolite in accordance with the present invention is zeolite CBV-100(trademark), a sodium aluminosilicate type zeolite. CVB-100(trademark) is a zeolite product available from Zeolyst International, Valley Forge, Pa. Other selected zeolites, and certain types of porous clays, such as montmorillonite clays, may be substituted for zeolite CBV-100(trademark) for particular applications. Useful clays must possess ion exchange characteristics as well as being able to act as molecular traps. The composite mix may be fabricated (pressed) into a thin, compacted foil, and the foil can be bent into a shallow cup-like shape. The presence of a small proportion of a surfactant, such as sodium lauryl sulfate, in the electrolyte as previously described improves the wetting of the molten electrolyte to the internal walls of the zeolite voids within the vanadium-zeolite composite separator element and thus helps to immobilize the molten electrolyte. While vanadium is mentioned as a preferred component of the separator element, other substances, such as the platinum-group metals and their alloys, refractory metals and their alloys, and the vanadium family (including vanadium, tantalum, and niobium) and their alloys, are equally applicable as components of the separator element of the present invention.
In still another embodiment of this invention, solid electrolyte may be incorporated into the composite mix which is then fabricated into a thin, compacted foil separator element. Mixing of the two or three components used in fabricating a separator element in accordance with the present invention may be accomplished by many means depending on the size of the batch. As mixing may be accomplished in a dry state, the most appropriate methods of mixing the separator materials include using a ball mill, a V-Shell Blender, or a ribbon blender. The most appropriate is a ball milling operation. Mixing is conducted on dry basis for a period of 30-45 minutes, using 0.25-5 inch diameter rubber balls. Mixing is accomplished by rolling a ceramic jar mill on a roller mill, with the mix and balls. This ensures intimate mixing of the materials. Once the mix is homogeneous, it is compacted into a thin casing (cup-like) foil around the anode.
For this embodiment of the invention, the relative proportions (by weight) of the three components may range from about 59-79% vanadium, 1-21% zeolite, and 10-30% electrolyte. A preferred mixture includes 75 wt. % vanadium, 17 wt. % zeolite, and 8 wt. % electrolyte. When such a vanadium/zeolite/electrolyte composite foil cup at least partially encloses the lithium-germanium alloy anode, for example, it provides added protection against migration of any free molten lithium, which could short out the circuit, as well as serving as an electron collectors. In a preferred embodiment of this aspect of the invention, the zeolite is zeolite CBV-100, a sodium aluminosilicate type zeolite, which is mixed with the solid electrolyte component in a ratio of about 7.5 parts by weight vanadium/1.7 parts by weight zeolite/0.8 parts by weight solid electrolyte. Other selected zeolites, and certain types of porous clays, such as montmorillonite clays, may be substituted for zeolite CBV-100 for particular applications. Useful clays must possess ion exchange characteristics as well as being able to act as molecular traps.
In an overall preferred embodiment, a thermal battery according to the present invention may include lithium/germanium alloy as the anode, vanadium pentoxide as the cathode, a ternary eutectic carbonate salt mix as the electrolyte, and with a vanadium metal-zeolite composite separator positioned between the heat pellet and the anode and/or cathode. In this preferred embodiment, the heat pellet is thermite. An MTV fuse roll surrounds the perimeter of the battery and contacts the thermite. The fuse roll igniter is a high precision micro-electromechanical system (MEMS) microcapillary initiator in accordance with this invention or, alternatively, a microelectric match.
A vent may be included on the battery housing to release pressure that accumulates beyond a predetermined level in the housing.
In another aspect, the present invention is directed to a thermal battery including: an anode, a cathode, and an electrolyte between the anode and cathode. The electrolyte has a first inactive state in which the electrolyte is electrically insulative, and has a second active state in which the electrolyte is electrically conductive between the anode and cathode. A heat element transforms the electrolyte from the first inactive state to the second active state.
In one embodiment, the first interface between the electrolyte and at least one of the anode and cathode is non-planar, for example, semi-spherical, elliptical, parabolic, or faceted in shape. A second interface between the electrolyte and the other of the anode and cathode is also non-planar, for example semi-spherical, elliptical, parabolic, or faceted in shape. The heat element is adjacent one of the anode and cathode and a third interface of the heat element and the one of the anode and cathode is non-planar. A second heat element adjacent the other of the anode and cathode and a fourth interface of the second heat element and the other of the anode and cathode is non-planar. The third and fourth interfaces are preferably semi-spherical, elliptical, parabolic, or faceted in shape.
The electrolyte is, for example, solid in the first inactive state and liquid in the second active state, and the heat element provides heat for transforming the electrolyte from the first inactive state to the second active state.
In another embodiment, a separator element may be included for preventing the flow of electrolyte in the second active state. In one example the separator element encompasses a base portion and side walls of the anode. The separator element may also include extension arms that extend beyond the side walls of the anode to ensure mechanical separation of the anode and cathode when the electrolyte is in the first inactive state.
In another embodiment, the separator element is positioned between two other components of said thermal battery, for example positioned between the electrolyte and the anode or the cathode, or positioned between the heat element and the anode or the cathode.
In another embodiment, the thermal battery is housed in a housing including a silicon-carbide (SiC) treated substrate, for example a silicon substrate or a silicon substrate treated with SiO2 prior to the SiC treatment. The housing is preferably hermetically sealed, and may include a microvent to release pressure that builds within the housing beyond a predetermined level.
In one embodiment, the housing includes a cavity etched in the substrate, within which the anode, cathode, electrolyte and heat element are deposited. Multiple cavities may be etched in the substrate, within which multiple unit cells, each unit cell including an anode, cathode, electrolyte and heat element, are deposited. Multiple unit cells may be stacked within the housing, each unit cell including an anode, cathode, electrolyte. IN this case adjacent unit cells in the stack may share a common heat element.
A pyrotechnic initiator may be employed for activating the heat element, and the heat element may include a pyrotechnic charge. The pyrotechnic charge may include a mixture of chemical components which produces an exothermic reaction upon being heated to ignition temperature, said exothermic reaction producing only reaction byproducts which are solids at ambient conditions, for example thermite.